Growth and grazing mortality rates of Prochlorococcus, Synechococcus and eukaryotic picophytoplankton in a bay of the Uwa Sea, Japan

Growth and grazing mortality rates of Synechococcus, Prochlorococcus and eukaryotic picophytoplankton were determined using dilution experiments in a bay of the Uwa Sea, Japan. Seasonal changes in the vertical distribution of each picophytoplankter were interpreted based on previous physico-chemical measurements and potential ecophysiological differences. Significant relationships between dilution and picophytoplankton growth rate were found only during the stratified period, suggesting that food linkage between the picophytoplankton and grazers was active during the stratified period. We found an almost 1:1 balance between growth and grazing mortality rates of Prochlorococcus and Synechococcus in the presence of both nanoflagellates and ciliates, suggesting that the abundance of the cyanobacteria is in a quasi-steady-state system with grazing losses compensated by growth. The relationship between growth and grazing mortality rate of eukaryotic picophytoplankton was unclear. We discuss the dynamics of each picophytoplankter abundance with special reference to the characteristics of the microbial food web in the bay, together with two physical events responsible for matter cycling within the bay.